Selenium rectifiers and method of manufacture



May 8, 1956 P. E UGHTY 2,745,047

SELENIUM RECTIFIERS AND METHOD OF MANUFACTURE Filed Dec. 14, 1951 NICKELSURF/ICED EL EC TRODE V NICKEL 2 \\\\\\w 5,455 METAL/ 5 COATED WITHF/RST LAYER 0F SELE/VIUM SELEN/UM 4 \XX HEAT TREATMENT OF 400%.,

0/? HIGHERJO FORM N/CKEL-SEL EN/DE NICKEL SEL EN/DE5 fg if A SEL ENIUM 6SELEN/UM CRYSTALS FORMED BY HEAT TREATMENT 7 Ulllllllllll COATED WITH ABARE/El? LAYER BARR/ER LAYER8- H WW7 COATED WITH A COUNTER- ELECTRODECOUNTER- ELECTRODE 9- mm 8 5 7 INVENTOR ATTO RN EY United States PatentO SELENIUM RECTIFIERS AND METHOD OF MANUFACTURE Paul E. Lighty,Lafayette, N. 1., assignor to International Telephone and TelegraphCorporation, a corporation of Maryland Application December 14, 1951,Serial No. 261,726

12 Claims. (Cl. 317-441) This invention relates to electrical rectifiersand more particularly to selenium rectifiers and methods ofmanufacturing same.

In the manufacture of selenium rectifiers, it is the practice heretoforeto first thoroughly roughen a base elec trode to obtain seleniumadherence, either by sand blasting or chemical etching, to nickel platethe roughened'surface, apply a layer of amorphous selenium, heat treatthe selenium coated electrode for a period of time sufficient to convertthe selenium to a crystalline state, apply a barrier or blocking film tothe selenium and apply thereto a counter-electrode. The rectifier thusproduced is electroformed by subjecting it to an electric current in thereverse direction to develop the blocking voltage and forwardconductivity characteristics of the rectifier.

The blocking voltage of a rectifier is defined as that A. C. voltagewhich may be applied to the rectifier without exceeding the maximumpermissible reverse current, and the maximum permissible reverse currentis determined by the permissible power dissipation, which is in turndictated by the highest temperature which the rectifier will withstandwithout deterioration.

Considerable effort has been made heretofore to increase this blockingvoltage without increasing the reverse current beyond a permissiblemaximum. While some improvement has been made over the past few years byvariation of ingredients and variation in the process steps employed,the best consistent blocking voltage obtainable heretofore was in theneighborhood of about 26 volts per cell. Any further improvement in suchblocking voltage characteristic carried with it a decrease of forwardconductivity and useful rectifier life.

One of the objects of this invention is to produce selenium rectifiershaving improved blocking voltage and forward conductivitycharacteristics which are measurably better than obtained bymanufacturing processes heretofore practiced.

Another object is to provide an improved method of producing seleniumrectifiers wherein not only the blocking voltage and forwardconductivity are greatly increased but improved selenium adherence tothe base electrode is also obtained.

Still another object is to provide a more eificient rectifier thanobtained heretofore; and a further object is to provide such a rectifierutilizing less selenium than required by manufacturing processesheretofore practiced.

One of the features of the invention is the manner in which greatlyincreased blocking voltage and forward conductivity are obtained. in themanufacture of selenium rectifiers, as heretofore practiced,particularly where the pressed powder method of applying amorphousselenium is used, the selenium layer is found to be inter-laced withmany fine cracks. It is believed that these cracks in the selenium layermay contribute to incipient short circuits or weak spots which must beburnt out before or during electroforining. The present inventioneliminates such cracks by applying a first selenium layer to a nickelsurfaced electrode which is then heated to an elevated 2,745,047Patented May 8,, 1956 ICC temperature sufiicient to form a layer ofnickel selenide of the interface portions of the nickel surface and thelayer of selenium. This compound of nickel selenide is formed in situ bysubjecting the electrode to a temperature of at least 400 C., theresulting nickel selenide providing a consistent and intimate bondbetween the selenium and the electrode surface. This improved bond alsorenders less critical the toughening operation of the base electrodeheretofore believed necessary for adher ence of the selenium layer. Thenickel selenide layer, by eliminating substantially the presence ofcracks or weak spots and by providing an intimate interface bond, makesfor higher forward conductivity and higher blocking voltages. Rectifierscontaining this nickel selenide layer also show indications of havinggood aging and long useful life characteristics.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent by reference tothe following de scription of an embodiment taken in conjunction withthe accompanying drawing, wherein the sole figure is a diagrammaticalillustration of a flow chart showing specimens, in exaggeratedproportions for illustration purposes, at different stages in theprocess of manufacturing a selenium rectifier.

In the drawing a base electrode 1 is shown surfaced at least on one sidethereof with a layer of nickel 2. The base electrode may comprise anysuitable conductor, such as iron, aluminum, magnesium, beryllium andvarious alloys thereof. The surface 3 of the base electrode ispreferably roughened by sand blasting or by chemical etching. While thistoughening has proved in prior practice to be a critical operation forinsuring proper adherence of the selenium layer, it is found not to becritical when the method for improving adherence of selenium ispracticed according to the present invention. The nickel surfacing maybe applied to the roughened surface by known plating techniques, such asdescribed in the paper entitled Some Chemical and Physical Problems inthe Manufacture of Selenium Rectifiers by C. A. Escoffery, Transactionsof the Electrochemical Society, vol. 90, 1946, pages 129-162.

The next operation, according to the present invention, is theapplication of a first layer of selenium 4 to the nickel surface 2. Thisthin layer or film of selenium may be applied by any one of a number ofmethods. For example, the selenium may be applied by heating theelectrode to a temperature above the melting point of selenium andapplying the selenium by rubbing a stick of the material across thesurface or by placing it in powder or pellet form on the heated surface.Another method of application is to spread selenium in the pellet orpowder form over the electrode and then to simultaneously press and heatthe selenium to effect an adherent layer. Still another method mayinvolve vaporization of the selenium to produce a selenium film on thenickel surface. Further, the coating maybe obtained by dipping theelectrode in a bath of selenium maintained slightly above the meltingtemperature of selenium. While material, usually selected from thehalogen group, is added to the selenium to improve its rectifyingcharacteristics, it is not necessary in the present invention to addsuch materials to the first application of selenium. Regardless ofwhether one of the methods referred to above or some 3 perature of 400C. or higher. This heating operation may be carried out in any one of anumber of different ways. For example, a flame may be applied to eitherthe coated or uncoated side of the electrode, the electrode may beplaced on a hot plate, or the electrode may be placed within an oven orfurnace. It is found important in this operation that the electrode beheated to a temperature of at least 400 C. in order to insure formationof a layer of a compound of nickel selenide 5 at the interface portionsof the nickel surface and the selenium layer. This heating operationshould be of a duration, depending on the maximum temperature used,sufficient to thin out the selenium until the entire nickel surface hasa continuous wetted appearance without the pres ence of bubbles. Thetemperature employed may exceed considerably the lower limit of 400 C.,the higher the temperature the shorter the heating interval. Fortemperatures ranging between 400 and 500 C., the duration of heating isnot critical, but where the temperature exceeds 500 C. the heatinginterval must be shortened to avoid excess evaporation of the selenium.T he formation of the nickel selenide in situ insures an intimate bondbetween the selenium and the nickel surface of the electrode. Thiscompound being of a metallic character and intimately bonded to theelectrode does not have the fine cracks that have been observed inselenium layers made by other processes.

Where a surplus of selenium is applied during the first application ofselenium, this surplus may be removed by spinning the electrode duringthe high temperature treatment. It is found, however, that where thepressed powder method is employed, such spinning is unneces saryproviding the selenium is reduced to a finely divided state suflicientto pass a 200 mesh screen. It is found however that spinning isdesirable to remove excessive selenium during the high temperaturetreatment when the particles are of the '80 mesh screen size.

Following the high temperature treatment, a second coating of amorphousselenium is applied. Any suitable method may be used, reference beingmade to the examples above mentioned. I have found, however, that ifselenium of the 200 mesh size is used rather than the coarse materialconventionally employed, a saving in the amount of selenium can be made.In actual practice of the present invention using 200 mesh particles,savings up to 30% can be achieved.

This second coating 6 of selenium may include other material, in theorder of a fraction of one percent, to improve the rectifyingcharacteristics of the final product, one such class of materials beingthe halides of selenium such as the mono-chloride, mono-bromide ormonoiodide. The reason why such addition material need not be includedin the first selenium layer is that the high heat treatment wouldundoubtedly drive off the addition agent, and furthermore, such additionagent is not essential to the formation of the adherent nickel selenidestrata 5.

The next operation is the known annealing operation wherein theelectrodes, coated as described above, are subjected to a heat treatmentof from 215 C. to 220 C. for about 15 to 35 minutes during which theamorphous selenium is converted to the crystalline state as indicated at7. Substantially all of the selenium contained in layer 6 and possiblyalso some of the selenium of layer 4 bonded to the nickel selenidestratum is converted to the crystalline state.

After conversion of the amorphous selenium to the crystalline state, theselenium surface is coated with a barrier layer or film 8 usuallyapplied in lacquer form. This barrier layer may be applied by any one ofa number of methods proposed in the prior art, but the preferred methodis believed to be the application of a lacquer such as disclosed in U.S. Patent No. 2,386,750, dated October 16, 1945, assigned to FederalTelephone and Radio Corporation. Following application of the barrierlayer, a counter-electrode material 9 is applied, such as for example, acadmium-bismuth-eutectic alloy.

The device thus produced is next subjected to an electroformingoperation as hereinbefore stated to develop the final blocking voltageand forward conductivity characteristics of the rectifier. The blockingvoltages of the rectifiers made in accordance with the present inventionhave ranged from about 36 volts to as high :as volts. For consistentproduction, blocking voltages in the lower portion of this range areassured. It is believed, however, that consistent high blocking voltagesof the order of 60 volts can be obtained by close control of theoperating steps herein outlined. The increased forward conductivity ofthe rectifier over rectifiers made by methods heretofore practiced isfound to be from 30 to 40%, thus resulting in greatly improvedefliciency. By practicing the present invention, it is found that asaving can be made of selenium through the formation of thinner seleniumlayers, and due to the higher efficiency of the cells, a smaller numberof cells are required for a given stack voltage.

While I have described above the principles of my invention inconnection with specific apparatus and method steps, it is to be clearlyunderstood that this description is made only by way of example and notas a limitation to the scope of my invention, as set forth in theobjects thereof and in the accompanying claims.

I claim:

1. In the method of making a selenium rectifier containing adjacentlayers of nickel and selenium, the step comprising producing a layer of.a compound of nickel selenide at the interface portions of the layersof nickel and selenium by heating said two layers to an elevatedtemperature of at least 400 C.

2. In the method according to claim 1, wherein said elevated temperatureis between about 400 C. and 500 C.

3. In the method of making a selenium rectifier, the steps comprisingapplying a layer of selenium to an electrode having a nickel surface,producing a layer of a compound of nickel selenide on said electrode byheating the selenium coated electrode to an elevated temperature of atleast 400 C. and spinning the electrode during the production of nickelselenide to thereby remove surplus selenium.

4. In the method of making a selenium rectifier, the steps comprisingapplying a layer of selenium to an electrode having a nickel surface,said selenium being in a finely divided state in the order of 200 meshsize, and producing a layer of a compound of nickel selen-ide on saidelectrode by heating the electrode to an elevated temperature of atleast 400 C.

5. In the method of making a selenium rectifier, the steps comprisingapplying a first layer of selenium to an electrode having a nickelsurface, heating the selenium coated electrode to an elevatedtemperature to produce a compound of nickel selenide of the interfaceportions of the nickel and selenium, applying a second layer ofamorphous selenium on said first layer, and annealing the electrode thuscoated to convert the selenium layer to a crystalline state.

6. In the method according to claim 5, wherein the step of heating ofthe first layer of selenium is brought to an elevated temperature of atleast 400 C.

7. In the method according to claim 5, wherein the selenium for saidsecond layer is selected in a finely divided state of the order of 200mesh size.

8. In the method according to claim 5, wherein the selenium for bothsaid firs-t and said second layers is selected in a finely divided stateof the order of 200 mesh size.

9. In the method of making a selenium rectifier, the steps comprisingapplying a first layer of amorphous selenium to a nickel coatedelectrode, heating the selenium coated electrode to an elevatedtemperature to produce a compound of nickel selenide of the interfaceportions of the nickel and selenium, applying a second layer ofamorphous selenium on said first layer, annealing the electrodethus'coated to convert the selenium to a crystalline state, applying abarrier layer to the selenium and coating over the barrier layer with alayer of counter-electrode material.

10. A rectifier comprising an electrode having a nickel surface, a layerof selenium and a layer of a compound of nickel selenide interposedbetween said surface and said layer of selenium.

11. A rectifier comprising an electrode having a nickel surface, a layerof selenium and a layer of a compound of nickel selenide formed in situfirom the interface portions of said nickel and said selenium.

12. A rectifier comprising an electrode having a nickel surface, a layerof selenium, a layer of nickel selenide bonding intimately the interfaceportions of said nickel surface and said layer of selenium, a barrierlayer dis- 5 posed on said selenium and a counter-electrode layerdisposed on said barrier layer.

References Cited in the file of this patent UNITED STATES PATENTS 102,433,353 Escofiery Dec. 30, 1947 2,446,237 Mueller Aug. 3,19482,504,226 R-au et a1. Apr. 18, 1950 2,524,270 'Pelfrey Oct. 3, 19502,644,915 Thurber et a1. July 7, 1953

1. IN THE METHOD OF MAKING A SELENIUM RECTIFIER CONTAINING ADJACENTLAYERS OF NICKEL AND SELENIUM, THE STEP COMPRISING PRODUCING A LAYER OFA COMPOUND OF NICKEL SELENIDE AT THE INTERFACE PORTIONS OF THE LAYERS OFNICKEL AND SELENIUM BY HEATING SAID TWO LAYERS TO AN ELEVATEDTEMPERATURE OF AT LEAST 400* C.